Protective coatings are generally used to protect the underlying material, the substrate, from damage due to direct exposure to outside environments. Historical protective coatings were detrimentally affected by differential dilation, i.e., a difference in expansion/contraction of two (or more) objects, so that dimensions that aligned at one time do not align at another time. This can be the result of mechanical loads, temperature gradients, or different coefficients of thermal expansion. Any or all of these causes might apply for coating/substrate interfaces. Induced stresses can cause the loss of bonding between the coating and the underlying material. When coatings are subjected to repeated heating and cooling cycles, thermally induced stresses and strains accumulate within the coating. The result is the loss of the coating in the regions that experience the greatest stresses, allowing direct exposure of the substrate material to the outside environment.
This phenomenon was studied early on and resulted in mechanical preparation techniques to resolve the problem. One approach was to prepare the material surface to enhance the bonding between the coating and the material to be protected by placing grooves in the material surface, as disclosed in H. S. Ingham and A. P. Shepard, Flame Spray Handbook, Vol. 1, Metco, Inc. (1964). The methods disclosed were techniques for inducing macro-roughness for bonding that was used for three primary purposes: (1) restrict shrink stresses, (2) increase bonding area, and (3) produce folds in laminations of the coating to increase the inherent strength.
Where thick coatings, 1/16 of an inch (1.6 mm) or greater, were applied to flat surfaces, problems of coating shrinkage and lifting at the edges were significant. One method of addressing the edge problem was to continue the sprayed coating over the edge at the termination of the flat surface. However, where this was not practical, Ingham et al., supra, suggest providing a row of grooves in the surface at the edge, or, preferably extending the grooves over the entire surface. Grooving parameters such as depth, width, and spacing are suggested. Referring to FIGS. 1a and 1b, the only grooving methods taught are open, round 20 or rectangular 30 grooves in material surface 10.
Several Patents have been issued detailing the use of these methods: U.S. Pat. No. 6,316,078, “Segmented Thermal Barrier Coating”, issued Nov. 13, 2001, to Smialek; and U.S. Pat. No. 5,558,922, “Thick Thermal Barrier Coating Having Grooves for Enhanced Strain Tolerance”, issued Sep. 24, 1996, to Gupta et al. Patent '078 claims a plurality of 3-dimensional features for preparing a surface for coating. Patent '922 teaches “an article . . . [with] first and second sets of grooves”; although these grooves are in the coating proper, the grooves serve to provide an avenue for reducing induced stresses. The 3-dimensional features of '078 and grooves in '922 are disclosed as open, rectangular grooves, i.e., the side walls of the grooves are perpendicular to the floor of the grooves.
The present invention was developed as a high-temperature plasma protective coating design, required to exhibit very good thermal conduction, for use in a high-temperature plasma environment. However, the present invention is not limited to thermal management coatings, but may also be applied to erosion or corrosion resistant coating applications.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.